Flt3 Ligand Bioactivity and Pharmacology in Neoplasia

Early-acting hematopoietic growth factors: biology and clinical experience

Secreted protein growth factors that stimulate the self-renewal, proliferation, and differentiation of the most primitive stem cells are among the most biologically interesting molecules and at least theoretically have diverse applications in the evolving field of regenerative medicine. Among this class of regulators, the early-acting hematopoietic growth factors and their cellular targets are perhaps the best characterized and serve as a paradigm for manipulating other stem cell based tissues. This chapter reviews the preclinical knowledge accumulated over ~40 years, since the discovery of the first such growth factor, and the clinical applications of those that, upon testing in humans, ultimately gained regulatory approval for the treatment of various hematological diseases.

Myeloid-derived suppressor cells in mammary tumor progression in FVB Neu transgenic mice

Female mice transgenic for the rat proto-oncogene c-erb-B2, under control of the mouse mammary tumor virus (MMTV) promoter (neuN), spontaneously develop metastatic mammary carcinomas. The development of these mammary tumors is associated with increased number of GR-1(+)CD11b(+) myeloid derived suppressor cells (MDSCs) in the peripheral blood (PB), spleen and tumor. We report a complex relationship between tumor growth, MDSCs and immune regulatory molecules in non-mutated neu transgenic mice on a FVB background (FVB-neuN). The first and second tumors in FVB-neuN mice develop at a median of 265 (147-579) and 329 (161-523) days, respectively, resulting in a median survival time (MST) of 432 (201 to >500) days. During tumor growth, significantly increased number of MDSCs is observed in the PB and spleen, as well as, in infiltrating the mammary tumors. Our results demonstrate a direct correlation between tumor size and the number of MDSCs infiltrating the tumor and an inverse relationship between the frequency of CD4(+) T-cells and MDSCs in the spleen. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) assessment of enzyme and cytokine transcript levels in the spleen, tumor, tumor-infiltrating non-parenchymal cells (NPCs) and mammary glands revealed a significant increase in transcript levels from grossly normal mammary glands and tumor-infiltrating NPCs during tumor progression. Tumor NPCs, as compared to spleen cells from wild-type (w/t) mice, expressed significantly higher levels of arginase-1 (ARG-1), nitric oxide synthase (NOS-2), vascular endothelial growth factor (VEGF-A) and significantly lower levels of interferon (IFN)-gamma, interleukin (IL)-2 and fms-like tyrosine kinase-3 ligand (Flt3L) transcript levels. Transcript levels in the spleens of tumor-bearing (TB) mice also differed from normal mice, although to a lesser extent than transcript levels from tumor-infiltrating NPCs. Furthermore, both spleen cells and NPCs from TB mice, but not control mice, suppressed alloantigen responses by syngeneic control spleen cells. Correlative studies revealed that the number of MDSCs in the spleen was directly associated with granulocyte colony stimulating factor (G-CSF) transcript levels in the spleen; while the number of MDSCs in the tumors was directly correlated with splenic granulocyte macrophage stimulating factor (GM-CSF) transcript levels, tumor volume and tumor cell number. Together our results support a role for MDSCs in tumor initiation and progressive, T-cell depression and loss of function provide evidence which support multiple mechanisms of MDSC expansion in a site-dependent manner.

Flt3 ligand delivered in a pluronic formulation prolongs the survival of mice with orthotopic pancreatic adenocarcinoma

Pancreatic adenocarcinoma is a devastating disease, characterized by asymptomatic development and extremely poor prognosis. Given the resistance of pancreatic cancer to standard chemo- and radiotherapy, we have focused on the development of immunotherapies for this disease. The number of dendritic cells (DCs), natural killer (NK) cells, and T-cells in the blood and secondary lymphoid organs is regulated by a group of hematopoietic growth factors, which includes fms-like tyrosine kinase-3 ligand (Flt3L). We have demonstrated previously that the bioavailability and in vivo half-life of Flt3L are increased by Flt3L formulation in the pluronic ProGelzx. In this study, we first examined the effectiveness of Flt3L delivered in ProGelz against subcutaneous (s.c.) pancreatic adenocarcinomas in mice. We found that an intramuscular (i.m.) injection of Flt3L in ProGelz significantly increased the survival of mice bearing s.c. pancreatic tumors, compared to the administration of phosphate-buffered saline (PBS) in ProGelz. We then tested Flt3L in ProGelz in an orthotopic pancreatic tumor model, and demonstrated that it significantly enhanced the survival of tumor-bearing mice, compared to PBS in ProGelz. Overall, these observations suggest that Flt3L formulated in ProGelz may have potential clinical utility as a treatment for pancreatic cancer.